This invention relates to a valve for use in breathing apparatus and more particularly to a demand valve.
From U.K. Patents Nos. 2 190 001, 2 195 900, 2 234 368 and 2 239 328 there is known a valve for use in breathing apparatus which includes a housing having a diaphragm mounted therein to define with the housing two chambers, a first one of which has a gas inlet for receiving breathing gas and a gas outlet through which breathing gas may be supplied to a face piece or mask for a user. The second chamber includes pivot means eccentrically mounted such that the diaphragm is pivoted at a position between its center of gravity and the gas inlet to the first chamber. The second chamber is vented direct to atmosphere, and , when a pressure sufficiently greater than atmospheric pressure is present in the first chamber, the diaphragm is pivoted to close the gas inlet against the pressure of the inflowing gas. A reduction of pressure in the first chamber when inhalation commences causes the diaphragm to pivot away from the gas inlet and allow gas to flow into the first chamber.
The known valve as described in outline above may be a demand valve per se or may be a pilot valve used in conjunction with a main valve as a pilot-operated demand valve. The first chamber of the known valve may additionally include spring means located at or near the gas inlet, the spring means exerting a biasing force on the diaphragm to ensure that the closing pressure required in the first chamber for pivoting the diaphragm to close the gas inlet is always sufficiently greater than the atmospheric pressure. Such a biasing spring is usually employed when the valve is a pilot valve and the pressure exerted on the diaphragm is a low pressure resulting from the small gas inlet to the first chamber which is the pilot jet.
The valves described in the aforesaid patents are all positive pressure valves because the pressure required in the first chamber to close the gas inlet is always greater than atmospheric pressure as discussed above. Positive pressure valves are used in protective respiratory systems where the user is isolated from the atmosphere by a face mask or other sealing means, and the positive pressure in the system ensures that any leakage past the sealing means is leakage from the protective system to atmosphere. However, in other applications, for example medical demand regulators, the outlet of the demand regulators is open to atmosphere for a substantial part of the time and if a valve according to the aforesaid patents were to be used for such an application, the positive pressure valve would deliver unrestricted flow to atmosphere.
Accordingly medical demand regulators require a negative pressure demand valve. The obvious modifications to the valve of the aforesaid U.K. patents in order to make this valve a negative pressure demand valve are either non-operative or unsatisfactory. In particular changing of the position of the spring in the first chamber to the opposite side of the pivot to the gas inlet will result in the pivot becoming inoperative, the reduction of pressure in the first chamber on inhalation simply sucking the diaphragm away from the pivot, and the gas inlet remaining closed. The same result will occur if the spring is positioned on the same side of the pivot as the gas inlet but on the opposite side of the diaphragm.
A further alternative possibility for making the valve of the aforesaid patents into a negative pressure demand valve is to change the positions of the pivot and the spring to the opposite sides of the diaphragm so that the pivot is in the first chamber and the spring is in the second chamber on the same side of the pivot as the gas inlet. Such a modification will not function satisfactorily since the normal level of pressure above atmospheric experienced during exhalation and communicated to the first chamber would lift the diaphragm off the pivot.